9.115 __ldrex intrinsic

The __ldrex intrinsic lets you load data from memory in your C or C++ code using an LDREX[size] instruction.

size in LDREX[size] is B for byte loads or H for halfword loads. If no size is specified, word loads are performed.


The compiler does not guarantee to preserve the state of the exclusive monitor. It might generate load and store instructions between the LDREX instruction generated for the __ldrex intrinsic and the STREX instruction generated for the __strex intrinsic. Because memory accesses can clear the exclusive monitor, code using the __ldrex and __strex intrinsics can have unexpected behavior. Where LDREX and STREX instructions are needed, ARM recommends using embedded assembly.


unsigned int __ldrex(volatile void *ptr)
points to the address of the data to be loaded from memory. To specify the type of the data to be loaded, cast the parameter to an appropriate pointer type.

Table 9-8 Access widths that the __ldrex intrinsic supports

Instruction Size of data loaded Pointer type
LDREXB byte unsigned char *
LDREXB byte signed char *
LDREXH halfword unsigned short *
LDREXH halfword signed short *
LDREX word int *

Return value

The __ldrex intrinsic returns the data loaded from the memory address pointed to by ptr.


The compiler does not recognize the __ldrex intrinsic when compiling for a target that does not support the LDREX instruction. The compiler generates either a warning or an error in this case, depending on the source language:
  • In C code: Warning: #223-D: function "__ldrex" declared implicitly.
  • In C++ code: Error: #20: identifier "__ldrex" is undefined.
The __ldrex intrinsic does not support access to doubleword data. The compiler generates an error if you specify an access width that is not supported.


int foo(void)
    int loc = 0xff;
    return __ldrex((volatile char *)loc);
Compiling this code with the command-line option --cpu=6k produces
||foo|| PROC
    MOV      r0,#0xff
    LDREXB   r0,[r0]
    BX       lr
Related reference
9.116 __ldrexd intrinsic
9.136 __strex intrinsic
9.137 __strexd intrinsic
Related information
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