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Id: elf_begin.3 4321 2025-12-22 20:48:10Z jkoshy
.Dd December 22, 2025 .Dt ELF_BEGIN 3 .Os .Sh NAME .Nm elf_begin .Nd open an ELF file or ar(1) archive .Sh LIBRARY .Lb libelf .Sh SYNOPSIS n libelf.h .Ft "Elf *" .Fn elf_begin "int fd" "Elf_Cmd cmd" "Elf *elf" .Sh DESCRIPTION Function .Fn elf_begin is used to open ELF files and .Xr ar 1 archives for further processing by other APIs in the .Xr elf 3 library. It is also used to access individual ELF members of an .Xr ar 1 archive in combination with the .Xr elf_next 3 and .Xr elf_rand 3 APIs.
p Argument .Fa fd is an open file descriptor returned from an .Xr open 2 system call. Function .Fn elf_begin uses argument .Fa fd for reading or writing depending on the value of argument .Fa cmd . Argument .Fa elf is primarily used for iterating through archives.
p The argument .Fa cmd can have the following values: l -tag -width "ELF_C_WRITE" t ELF_C_NULL Causes .Fn elf_begin to return .Dv NULL . Arguments .Fa fd and .Fa elf are ignored, and no additional error is signalled. t ELF_C_READ This value is to be when the application wishes to examine (but not modify) the contents of the file specified by the arguments .Fa fd and .Fa elf . It can be used for both .Xr ar 1 archives and for ELF objects.
p If argument .Fa elf is .Dv NULL , the library will allocate a new ELF descriptor for the file being processed. The argument .Fa fd should have been opened for reading.
p If argument .Fa elf is not .Dv NULL , and references a regular ELF file previously opened with .Fn elf_begin , then the activation count for the descriptor referenced by argument .Fa elf is incremented. The value in argument .Fa fd should match that used to open the descriptor argument .Fa elf .
p If argument .Fa elf is not .Dv NULL , and references a descriptor for an .Xr ar 1 archive opened earlier with .Fn elf_begin , a descriptor for an element in the archive is returned as described in the section .Sx "Processing ar(1) archives" below. The value for argument .Fa fd should match that used to open the archive earlier.
p If argument .Fa elf is not .Dv NULL , and references an .Xr ar 1 archive opened earlier with .Fn elf_memory , then the value of the argument .Fa fd is ignored. t Dv ELF_C_RDWR This command is used to prepare an ELF file for reading and writing. This command is not supported for .Xr ar 1 archives.
p Argument .Fa fd should have been opened for reading and writing. If argument .Fa elf is .Dv NULL , the library will allocate a new ELF descriptor for the file being processed. If the argument .Fa elf is non-null, it should point to a descriptor previously allocated with .Fn elf_begin with the same values for arguments .Fa fd and .Fa cmd ; in this case the library will increment the activation count for descriptor .Fa elf and return the same descriptor.
p Changes to the in-memory image of the ELF file may be written back to disk using the .Xr elf_update 3 function. t Dv ELF_C_WRITE This command is used when the application wishes to create a new ELF file. Argument .Fa fd should have been opened for writing. Argument .Fa elf is ignored, and the previous contents of file referenced by argument .Fa fd are overwritten. .El .Ss Processing ar(1) archives An .Xr ar 1 archive may be opened in read mode (with argument .Fa cmd set to .Dv ELF_C_READ ) using .Fn elf_begin or .Fn elf_memory . The returned ELF descriptor can be passed into to subsequent calls to .Fn elf_begin to access individual members of the archive.
p Random access within an opened archive is possible using the .Xr elf_next 3 and .Xr elf_rand 3 functions.
p The symbol table of the archive may be retrieved using .Xr elf_getarsym 3 . .Sh RETURN VALUES The function returns a pointer to a ELF descriptor if successful, or .Dv NULL if an error occurred. .Sh EXAMPLES To iterate through the members of an .Xr ar 1 archive, use: d -literal -offset indent Elf_Cmd c; Elf *ar_e, *elf_e; ... c = ELF_C_READ; if ((ar_e = elf_begin(fd, c, (Elf *) 0)) == 0) { ... handle error in opening the archive ... } while ((elf_e = elf_begin(fd, c, ar_e)) != 0) { ... process member referenced by elf_e here ... c = elf_next(elf_e); elf_end(elf_e); } .Ed
p To create a new ELF file, use: d -literal -offset indent int fd; Elf *e; ... if ((fd = open("filename", O_RDWR|O_TRUNC|O_CREAT, 0666)) < 0) { ... handle the error from open(2) ... } if ((e = elf_begin(fd, ELF_C_WRITE, (Elf *) 0)) == 0) { ... handle the error from elf_begin() ... } ... create the ELF image using other elf(3) APIs ... elf_update(e, ELF_C_WRITE); elf_end(e); .Ed .Sh ERRORS Function .Fn elf_begin can fail with the following errors: l -tag -width "[ELF_E_RESOURCE]" t Bq Er ELF_E_ARCHIVE The archive denoted by argument .Fa elf could not be parsed. t Bq Er ELF_E_ARGUMENT The value in argument .Fa cmd was unrecognized. t Bq Er ELF_E_ARGUMENT A non-null value for argument .Fa elf was specified when .Fa cmd was set to .Dv ELF_C_RDWR . t Bq Er ELF_E_ARGUMENT The value of argument .Fa fd differs from the one the ELF descriptor .Fa elf was created with. t Bq Er ELF_E_ARGUMENT Argument .Fa cmd differs from the value specified when ELF descriptor .Fa elf was created. t Bq Er ELF_E_ARGUMENT An .Xr ar 1 archive was opened with .Fa cmd set to .Dv ELF_C_RDWR . t Bq Er ELF_E_ARGUMENT The file referenced by argument .Fa fd was empty. t Bq Er ELF_E_ARGUMENT The underlying file for argument .Fa fd was of an unsupported type. t Bq Er ELF_E_ARGUMENT An invalid file descriptor value was used for argument .Fa fd . t Bq Er ELF_E_IO An IO error occurred when attempting to use the file descriptor value in argument .Fa fd . t Bq Er ELF_E_RESOURCE An out of memory condition was encountered. t Bq Er ELF_E_SEQUENCE Function .Fn elf_begin was called before a working version was established with .Xr elf_version 3 . t Bq Er ELF_E_VERSION The ELF object referenced by argument .Fa fd was of an unsupported ELF version. .El .Sh SEE ALSO .Xr elf 3 , .Xr elf_end 3 , .Xr elf_errno 3 , .Xr elf_memory 3 , .Xr elf_next 3 , .Xr elf_open 3 , .Xr elf_rand 3 , .Xr elf_update 3 , .Xr gelf 3
Indexes created Sat Feb 28 05:31:39 UTC 2026